Generating three-dimensional objects

ABSTRACT

At least one agent distributor may be to selectively deliver coalescing agent onto portions of a layer of build material at a first density and at a second density lower than the first density. A controller may be to control the at least one agent distributor to selectively deliver the coalescing agent at the first and second densities onto respective first and second portions of the layer in respective first and second patterns derived from data representing the three-dimensional object to be generated, so that when energy is applied to the layer the build material may coalesce and solidify to form a slice of the three-dimensional object in accordance with the first pattern. The second portion may be in proximity to a boundary of the first portion. Presence of the coalescing agent in the second portion may be to prevent at least some heat from flowing away from the first portion when the energy is applied.

BACKGROUND

Additive manufacturing systems that generate three-dimensional objectson a layer-by-layer basis have been proposed as a potentially convenientway to produce three-dimensional objects. The quality of objectsproduced by such systems may vary widely depending on the type ofadditive manufacturing technology used.

BRIEF DESCRIPTION

Some examples are described with respect to the following figures:

FIG. 1a illustrates a system for generating a three-dimensional objectaccording to some examples;

FIG. 1b is a flow diagram illustrating a method according to someexamples;

FIG. 1c is a block diagram illustrating a non-transitory computerreadable storage medium (CRM) according to some examples;

FIG. 2 is a simplified isometric illustration of an additivemanufacturing system according to some examples;

FIG. 3 is a flow diagram illustrating a method of generating athree-dimensional object according to some examples;

FIGS. 4a-c show data representing a three-dimensional object accordingto some examples;

FIGS. 5a-d show a series of cross-sectional side views of layers ofbuild material according to some examples; and

FIGS. 6a-d show a series of top views of layers of build materialaccording to some examples.

DETAILED DESCRIPTION

The following terminology is understood to mean the following whenrecited by the specification or the claims. The singular forms “a,”“an,” and “the” mean “one or more,” The terms “including” and “having”are intended to have the same inclusive meaning as the term“comprising.”

Some additive manufacturing systems generate three-dimensional objectsthrough the solidification of portions of successive layers of buildmaterial, such as a powdered or liquid build material. The properties ofgenerated objects may be dependent on the type of build material and thetype of solidification mechanism used, In some examples, solidificationmay be achieved using a liquid binder agent to chemically solidify buildmaterial, In other examples, solidification may be achieved by temporaryapplication of energy to the build material. This may, for example,involve use of a coalescing agent, which is a material that, when asuitable amount of energy is applied to a combination of build materialand coalescing agent, may cause the build material to coalesce andsolidify. For example, the coalescing agent may act as an absorber ofapplied energy such that the portions of build material havingcoalescing agent experience coalescence and solidification. In someexamples, a multiple agent additive manufacturing system may be usedsuch as that described in PCT Application No. PCT/EP2014/050841 filed onJan. 16, 2014, entitled “GENERATING A THREE-DIMENSIONAL OBJECT”, theentire contents of which are hereby incorporated herein by reference.For example, in addition to selectively delivering coalescing agent tolayers build material, coalescence modifier agent may also beselectively delivered to layers of build material. A coalescencemodifier agent may serve to modify the degree of coalescence of aportion of build material on which the coalescence modifier agent hasbeen delivered or has penetrated. In yet other examples, other methodsof solidification may be used, for example selective laser sintering(SLS), light polymerization, among others. The examples described hereinmay be used with any of the above additive manufacturing systems andsuitable adaptations thereof.

In examples in which solidification is achieved using coalescing agentand application of energy, energy absorbed by build material on whichcoalescing agent has been delivered or has penetrated to form the objectmay also partially propagate away from the object being generated andinto surrounding build material on which coalescing agent has not beendelivered and in which solidification is not intended. This effect maybe exacerbated when using build materials that may have relatively highheat conductivity, as this may cause prevent formation of a heatreservoir beneath the surface of each newly created layer as it isformed. The heat in the reservoir may then propagate away from theobject being generated, e.g. laterally across the build material, belowthe newest layer, and/or into a future layer once it is applied on thenewest layer.

Thus, the object being generated may receive less heat than intended andtherefore experience a lesser degree of coalescence and solidificationthan intended, e.g. may not completely coalesce and solidify asintended, resulting in poor object properties, such as poor surfaceproperties, accuracy, strength, or inter-layer bonding. In someexamples, thin portions of the object being generated may be especiallyat risk for under-solidification as heat may propagate away from thethin portions. This may cause the thin portions to be incompletelyformed and may even cause these thin portions to break off after theobject is generated.

Accordingly, the present disclosure provides various examples that maye.g. result in objects with good object properties, including goodsurface properties, accuracy, strength, and inter-layer bonding. Forexample, thin portions of objects may accurately form and may not breakoff. This may, for example, be achieved by providing coalescing agent ata first density in a first portion that is to be solidified, and acoalescing agent at a second density less than the first density in asecond portion surrounding the first portion, e.g. surrounding a thinportion of the first portion. The second density may allow the secondportion to heat up such that it may prevent heat from flowing away fromthe first portion (e.g. thin portion), but may not be high enough toallow the second portion to achieve full solidification.

FIG. 1 a is a block diagram illustrating a system 100 for generating athree-dimensional object according to some examples. The system 100 mayinclude at least one agent distributor 102 to selectively delivercoalescing agent onto portions of a layer of build material at a firstdensity and at a second density lower than the first density. The system100 may include a controller 104 to control the at least one agentdistributor to selectively deliver the coalescing agent at the first andsecond densities onto respective first and second portions of the layerin respective first and second patterns derived from data representingthe three-dimensional object to be generated, so that when energy isapplied to the layer the build material may coalesce and solidify toform a slice of the three-dimensional object in accordance with thefirst pattern. The second portion may be in proximity to a boundary ofthe first portion. Presence of the coalescing agent in the secondportion may be to prevent at least some heat from flowing away from thefirst portion when the energy is applied.

FIG. 1b is a flow diagram illustrating a method 110 according to someexamples. At 112, a layer of build material may be delivered. At 114,coalescing agent may be selectively deposited to a first portion of thelayer at a first density. At 116, coalescing agent may be selectivelydeposited to a second portion of the layer at a second density lowerthan the first density. The second portion may be disposed around aboundary of the first portion. At 118, energy may be applied to thelayer to cause the first portion to coalesce and solidify to form aslice of a three-dimensional object. Presence of coalescing agent in thesecond portion may prevent at least some heat from flowing away from thefirst portion.

FIG. 1c is a block diagram illustrating a non-transitory computerreadable storage medium (CRM) 120 according to some examples. The medium120 may include instructions 122 that when executed by a processor,cause the processor to obtain data representing a three-dimensionalobject to be generated. The data may include a first portion definingwhere coalescing agent is to be delivered at a first density. The medium120 may include instructions 124 that when executed by a processor,cause the processor to modify the data to include a second portion onwhich coalescing agent is to be delivered at a second density less thanthe first density. The second portion may surround at least part of thefirst portion. The medium 120 may include instructions 126 that whenexecuted by a processor, cause the processor to control, using themodified data, at least one agent distributor to deliver the coalescingagent to the first portion at a first density and to the second portionat a second density such that the first portion is to coalesce andsolidify when energy is applied. The second density may be insufficientto achieve full solidification in the second portion when the energy isapplied. Presence of the coalescing agent in the second portion may beto prevent heat from flowing away from the first portion when the energyis applied.

FIG. 2 is a simplified isometric illustration of an additivemanufacturing system 200 according to some examples. The system 200 maybe operated as described further below with reference to the flowdiagram of FIG. 3 to generate a three-dimensional object.

In some examples the build material may be a powder-based buildmaterial. As used herein the term powder-based materials is intended toencompass both dry and wet powder-based materials, particulatematerials, and granular materials. In some examples, the build materialmay include a mixture of air and solid polymer particles, for example ata ratio of about 40% air and about 60% solid polymer particles. Onesuitable material may be Nylon 12, which is available, for example, fromSigma-Aldrich Co. LLC. Another suitable Nylon 12 material may be PA 2200which is available from Electro Optical Systems EOS GmbH. Other examplesof suitable build materials may include, for example, powdered metalmaterials, powdered composite materials, powdered ceramic materials,powdered glass materials, powdered resin material, powdered polymermaterials, and the like, and combinations thereof. It should beunderstood, however, that the examples described herein are not limitedto powder-based materials or to any of the materials listed above. Inother examples the build material may be in the form of a paste, liquidor a gel. According to one example a suitable build material may be apowdered semi-crystalline thermoplastic material.

The additive manufacturing system 200 may include a system controller210. Any of the operations and methods disclosed herein may beimplemented and controlled in the additive manufacturing system 200and/or controller 210.

The controller 210 may include a processor 212 for executinginstructions that may implement the methods described herein. Theprocessor 212 may, for example, be a microprocessor, a microcontroller,a programmable gate array, an application specific integrated circuit(ASIC), a computer processor, or the like. The processor 212 may, forexample, include multiple cores on a chip, multiple cores acrossmultiple chips, multiple cores across multiple devices, or combinationsthereof. In some examples, the processor 212 may include at least oneintegrated circuit (IC), other control logic, other electronic circuits,or combinations thereof.

The processor 212 may be in communication with a computer-readablestorage medium (CRM) 216, e.g. via a communication bus. Thecomputer-readable storage medium (CRM) 216 may include a single mediumor multiple media. For example, the computer readable storage medium(CRM) 216 may include one or both of a memory of the ASIC, and aseparate memory in the controller 210. The computer readable storagemedium 216 may be any electronic, magnetic, optical, or other physicalstorage device. For example, the computer-readable storage medium 216may be, for example, random access memory (RAM), static memory,read-only memory, an electrically erasable programmable read-only memory(EEPROM), a hard drive, an optical drive, a storage drive, a CD, a DVD,and the like. The computer-readable storage medium 216 may benon-transitory. The computer-readable storage medium 216 may store,encode, or carry computer executable instructions 218 that, whenexecuted by the processor 212, may cause the processor 212 to performany of the methods or operations disclosed herein according to variousexamples.

The system 200 may include a coalescing agent distributor 202 toselectively deliver coalescing agent to successive layers of buildmaterial provided on a support member 204. According to one non-limitingexample, a suitable coalescing agent may be an ink-type formulationcomprising carbon black, such as, for example, the ink formulationcommercially known as CM997A available from Hewlett-Packard Company. Inone example such an ink may additionally comprise an infra-red lightabsorber. In one example such an ink may additionally comprise a nearinfra-red light absorber. In one example such an ink may additionallycomprise a visible light absorber. In one example such an ink mayadditionally comprise a UV light absorber. Examples of inks comprisingvisible light enhancers are dye based colored ink and pigment basedcolored ink, such as inks commercially known as CM993A and CE042Aavailable from Hewlett-Packard Company.

The controller 210 may control the selective delivery of coalescingagent to a layer of provided build material in accordance with theinstructions 218.

The agent distributor 202 may be a printhead, such as a thermal inkjetprinthead or a piezo inkjet printhead. The printhead may have arrays ofnozzles. In one example, printheads such as those commonly used incommercially available inkjet printers may be used. In other examples,the agents may be delivered through spray nozzles rather than throughprintheads. Other delivery mechanisms may be used as well. The agentdistributor 202 may be used to selectively deliver, e.g. deposit,coalescing agent when in the form of suitable fluids such as a liquid.

The coalescing agent distributor 202 may include a supply of coalescingagent or may be connectable to a separate supply of coalescing agent.

The agent distributor 202 may be used to selectively deliver, e.g.deposit, coalescing agent when in the form of a suitable fluid such asliquid. In some examples, the agent distributor 202 may have an array ofnozzles through which the agent distributor 202 is able to selectivelyeject drops of fluid. In some examples, each drop may be in the order ofabout 10 pica liters (pl) per drop, although in other examples the agentdistributor 202 is able to deliver a higher or lower drop size. In someexamples the agent distributor 202 is able to deliver variable sizedrops,

In some examples the coalescing agent may comprise a liquid carrier,such as water or any other suitable solvent or dispersant, to enable itto be delivered via a printhead.

In some examples the printheads may be drop-on-demand printhead. Inother examples the printhead may be continuous drop printhead.

In some examples, the agent distributor 202 may be an integral part ofthe system 200. In some examples, the agent distributor 202 may be userreplaceable, in which case they may be removably insertable into asuitable agent distributor receiver or interface module of the system200.

In the example illustrated in FIG. 2, the agent distributor 202 may havea length that enables it to span the whole width of the support member204 in a so-called page-wide array configuration. In one example thismay be achieved through a suitable arrangement of multiple printheads.In other examples a single printhead having an array of nozzles having alength to enable them to span the width of the support member 204 may beused. In other examples, the agent distributor 202 may have a shorterlength that does not enable it to span the whole width of the supportmember 204.

The agent distributor 202 may be mounted on a moveable carriage toenable it to move bi-directionally across the length of the support 204along the illustrated y-axis. This enables selective delivery ofcoalescing agent across the whole width and length of the support 204 ina single pass. In other examples the agent distributor 202 may be fixed,and the support member 204 may move relative to the agent distributor202.

In other examples the agent distributors may be fixed, and the supportmember 204 may move relative to the agent distributors.

It should be noted that the term ‘width’ used herein is used togenerally denote the shortest dimension in the plane parallel to the xand y axes illustrated in FIG. 2, whilst the term ‘length’ used hereinis used to generally denote the longest dimension in this plane.However, it will be understood that in other examples the term ‘width’may be interchangeable with the term ‘length’. For example, in otherexamples the agent distributor 202 may have a length that enables themto span the whole length of the support member 204 whilst the moveablecarriage may move bi-directionally across the width of the supportmember 204.

In another example the agent distributor 202 does not have a length thatenables it to span the whole width of the support member but areadditionally movable bi-directionally across the width of the supportmember 204 in the illustrated x-axis. This configuration enablesselective delivery of coalescing agent across the whole width and lengthof the support 204 using multiple passes. Other configurations, however,such as a page-wide array configuration, may enable three-dimensionalobjects to be created faster.

The system 200 may further comprise a build material distributor 224 toprovide, e.g. deliver and/or deposit, successive layers of buildmaterial on the support member 204. Suitable build material distributors224 may include, for example, a wiper blade and a roller. Build materialmay be supplied to the build material distributor 224 from a hopper orbuild material store. In the example shown the build materialdistributor 224 moves across the length (y-axis) of the support member204 to deposit a layer of build material. As previously described, alayer of build material will be deposited on the support member 204,whereas subsequent layers of build material will be deposited on apreviously deposited layer of build material. The build materialdistributor 224 may be a fixed part of the system 200, or may not be afixed part of the system 200, instead being, for example, a part of aremovable module. In some examples, the build material distributor 224may be mounted on a carriage.

In some examples, the thickness of each layer may have a value selectedfrom the range of between about 50 to about 300 microns, or about 90 toabout 110 microns, or about 250 microns, although in other examplesthinner or thicker layers of build material may be provided. Thethickness may be controlled by the controller 210, for example based onthe instructions 218.

In some examples, there may be any number of additional agentdistributors and build material distributors relative to thedistributors shown in FIG. 2. In some examples, the distributors ofsystem 200 may be located on the same carriage, either adjacent to eachother or separated by a short distance. In other examples, two or morecarriages each may contain a distributor. For example, each distributormay be located in its own separate carriage. Any additional distributorsmay have similar features as those discussed earlier with reference tothe coalescing agent distributor 202. However, in some examples,different agent distributors may deliver different coalescing agentsand/or coalescence modifier agents, for example.

In the example shown the support 204 is moveable in the z-axis such thatas new layers of build material are deposited a predetermined gap ismaintained between the surface of the most recently deposited layer ofbuild material and lower surface of the agent distributor 202. In otherexamples, however, the support 204 may not be movable in the z-axis andthe agent distributor 202 may be movable in the z-axis.

The system 200 may additionally include an energy source 226 to applyenergy to build material to cause the solidification of portions of thebuild material according to where coalescing agent has been delivered orhas penetrated. In some examples, the energy source 226 is an infra-red(IR) radiation source, near infra-red radiation source, halogenradiation source, or a light emitting diode. In some examples, theenergy source 226 may be a single energy source that is able touniformly apply energy to build material deposited on the support 204.In some examples, the energy source 226 may comprise an array of energysources.

In some examples, the energy source 226 is configured to apply energy ina substantially uniform manner to the whole surface of a layer of buildmaterial. In these examples the energy source 226 may be said to be anunfocused energy source. In these examples, a whole layer may haveenergy applied thereto simultaneously, which may help increase the speedat which a three-dimensional object may be generated.

In other examples, the energy source 226 is configured to apply energyin a substantially uniform manner to a portion of the whole surface of alayer of build material. For example, the energy source 226 may beconfigured to apply energy to a strip of the whole surface of a layer ofbuild material. In these examples the energy source may be moved orscanned across the layer of build material such that a substantiallyequal amount of energy is ultimately applied across the whole surface ofa layer of build material.

In some examples, the energy source 226 may be mounted on the moveablecarriage 203 a or 203 b.

In other examples, the energy source 226 may apply a variable amount ofenergy as it is moved across the layer of build material, for example inaccordance with instructions 218. For example, the controller 210 maycontrol the energy source only to apply energy to portions of buildmaterial on which coalescing agent has been applied.

In further examples, the energy source 226 may be a focused energysource, such as a laser beam. In this example the laser beam may becontrolled to scan across the whole or a portion of a layer of buildmaterial. In these examples the laser beam may be controlled to scanacross a layer of build material in accordance with agent deliverycontrol data. For example, the laser beam may be controlled to applyenergy to those portions of a layer of on which coalescing agent isdelivered.

The combination of the energy supplied, the build material, and thecoalescing agent may be selected such that, excluding the effects of anycoalescence bleed: i) portions of the build material on which nocoalescing agent have been delivered do not coalesce when energy istemporarily applied thereto; ii) portions of the build material on whichonly coalescing agent has been delivered or has penetrated coalesce whenenergy is temporarily applied thereto do coalesce.

Although not shown in FIG. 2, in some examples the system 200 mayadditionally comprise a pre-heater to maintain build material depositedon the support member 204 within a predetermined temperature range. Useof a pre-heater may help reduce the amount of energy that has to beapplied by the energy source 226 to cause coalescence and subsequentsolidification of build material on which coalescing agent has beendelivered or has penetrated.

FIG. 3 is a flow diagram illustrating a method 300 of generating athree-dimensional object according to some examples. In some examples,the orderings shown may be varied, some elements may occursimultaneously, some elements may be added, and some elements may beomitted.

In describing FIG. 3, reference will be made to FIGS. 2, 4 a-c, 5 a-d,and 6 a-d. FIGS. 4a-c show data representing a three-dimensional objectaccording to some examples. FIGS. 5a-d show a series of cross-sectionalside views of layers of build material according to some examples. FIGS.6a-d show a series of top views of layers of build material according tosome examples. A top view of layers along line 6 a-6 a of FIG. 5a isshown in FIG. 6a , and a cross sectional side view along lines 5 a-a ofFIG. 6a is shown in FIG. 5a . A top view of layers along line 6 b-6 b ofFIG. 5b is shown in FIG. 6b , and a cross sectional side view alonglines 5 b-b of FIG. 6b is shown in FIG. 5b . A top view of layers alongline 6 c-6 c of FIG. 5c is shown in FIG. 6c , and a cross sectional sideview along lines 5 c-c of FIG. 6c is shown in FIG. 5c . A top view oflayers along line 6 d-6 d of FIG. 5d is shown in FIG. 6d , and a crosssectional side view along lines 5 d-d of FIG. 6a is shown in FIG. 5 d.

At 302, data 400 representing the three dimensional object may begenerated or obtained by the controller 210. “Data representing thethree dimensional object” is defined herein to include any data definingthe object from its initial generation as three dimensional objectdesign data to its conversion into slice data representing slices of theobject to be generated. The data 400 may be part of the instructions218.

The three-dimensional object design data may represent athree-dimensional model of an object to be generated, and/or propertiesof the object (e.g. density, surface roughness, strength, and the like).The model may define the solid portions of the object. Thethree-dimensional object design data may be received, for example, froma user via an input device 220, as input from a user, from a softwaredriver, from a software application such as a computer aided design(CAD) application, or may be obtained from a memory storing default oruser-defined object design data and object property data. Thethree-dimensional object design data may be processed by athree-dimensional object processing system to generate slice datarepresenting slices of parallel planes of the model.

Each slice may define a portion of a respective layer of build materialthat is to be solidified by the additive manufacturing system. The slicedata may undergo transformations from (1) vector slice data representingslices of the object in a vector format, to (2) contone slice datarepresenting slices of the object in a bitmap or rasterized format, to(3) halftone slice data representing locations or patterns in whichdrops of agent are to be deposited on a layer of build material for eachslice of the object, to (4) mask slice data representing the timing ofwhen drops of agent are to be deposited in locations, portions, orpatterns on a layer of build material for each slice of the object, e.g.using nozzles of an agent distributor.

In the example of FIG. 4a , an example of data 400 is shown as slicedata having a slice 402 of the object to be generated. The slice 402 mayinclude a thick portion 404. A thick portion is a portion that does nothave a thickness across its axes (e.g. width or length) that is lessthan a threshold thickness. The slice 402 may include a thin portion406. A thin portion is a portion that has a thickness across at leastone axis (e.g. width or length) that is below the threshold thickness.In some examples, the threshold thickness may be 2 millimeters, or maybe 1 millimeter.

In some examples, thin portions of slices may be more susceptible tounder-solidification than thick portions of slices due to heatdissipation from the thin portion to surrounding build material, asdiscussed earlier. Therefore, the data 400 may be processed at 304 to308 to add a portion 414 (as shown in FIG. 4c ) in which a lower densityor amount of coalescing agent is to be delivered relative to the densityor amount of coalescing agent to be delivered to generate the slice 402.The portion 414 may surround the boundary (e.g. external boundary) ofany thin portions, e.g. thin portion 406.

In some examples, rather than adding the portion 414 around the boundaryof thin portions, the portion 414 may be added around the entirecircumferential boundary of the entire slice including thin and thickportions, such that the portion 414 completely surrounds the slice 402.

The method 300 at 304 to 308 describes the data 400 being processed, butin other examples other types of data may be processed at 304 to 308including three-dimensional object design data or any other type ofslice data. In addition, if slice data is processed at 304 to 308, thenall slice data (representing multiple slices of the objet) of a printjob may be processed,

At 304, the controller 210 may erode the data 400 to generate erodeddata 408 as shown in FIG. 4b . The erosion may involve eroding (e.g.shrinking) the boundaries of the slice 402 in the data 400 by apredetermined eroding distance. In some examples, the eroding distancemay be equal to half the threshold thickness used to identify thinportions. For example, if the threshold thickness is 2 millimeters, thenthe eroding distance may be 1 millimeter. Thus, for example, once theslice 402 is eroded, any thin portion that is below the thresholdthickness (e.g. 2 millimeters) may be removed in the eroded data 408because eroding both of opposite-facing boundaries by the erodingdistance (e.g. 1 millimeter) may erode a thin portion by the thresholdthickness of e.g. 2 millimeters, In FIG. 4b , the eroded slice 410 isshown with no thin portions (e.g. the thin potion 406 is removed), whileretaining an eroded thick portion 410 derived from the thick portion404.

At 306, the controller 210 may compare the data 400 and eroded data 402to identify any thin portions in the data 400 e.g. the thin portion 406.This may be done by checking which portions of the data 400 have beenremoved completely in the eroded data 402. Thus, for example, thecontroller 210 may identify the thin portion 402 as having been removedby the erosion.

At 308, the controller 210 may modify the data 400 to generate modifieddata 412. In addition to including the slice 402 of data 400, themodified data 412 may include portion 414 in which a lower density oramount of coalescing agent is to be delivered relative to the density oramount of coalescing agent to be delivered to generate the slice 402.The portion 414 may surround the boundary of the thin portion 406. Themodification may be done by dilating the identified thin portion 406 (orin examples where the entire slice is to be surrounded by the portion414, the entire slice 402 may be dilated). The dilation may involvedilating (e.g. expanding) the boundaries of the thin portion 406 (orentire slice 402 if the entire slice 402 is to be surrounded by theportion 414) by a predetermined dilating distance. In some examples, thedilating distance may be equal to about 0.1 millimeters to about 10millimeters. The dilating distance may correspond to the width of theportion 414 adjacent to the thin portion 406. In some examples, thedilating distance may depend on the thickness or the portion (e.g. thinportion). For example, a first thin portion of the object that isthinner than a second thin portion of the object may be dilated to agreater dilating distance than the second thin portion. Once dilation ofcomplete, the modified data 412 may be compared to the data 400 toidentify the newly added portion. The newly added portion may bedesigned as the portion 414 in which a lower density or amount ofcoalescing agent is to be delivered.

As discussed earlier, if slice data is processed at 304 to 308, theprocessing of 304 to 308 may be performed for all slice data(representing multiple slices of the object) of a print job. In someexamples, slice data representing separate slices may be processedduring printing in 310 to 314, e.g. slice data representing theparticular slice to be printed in the particular iteration of 310 to 314may be processed e.g. before applying the agent at 312.

At 310, a layer 502 b of build material may be provided, as shown inFIGS. 5a and 6a . For example, the controller 210 may control the buildmaterial distributor 224 to provide the layer 502 b on a previouslycompleted layer 502 a on the support member 204 by causing the buildmaterial distributor 224 to move along the y-axis as discussed earlier.The completed layer 502 a may include a solidified portion 506. Althougha completed layer 502 a is shown in FIGS. 5a-d for illustrativepurposes, it is understood that 310 to 314 may initially be applied togenerate the first layer 502 a.

In some examples, after applying the layer 502 b, the layer 502 b ofbuild material may be heated by the heater to heat and/or maintain thebuild material within a predetermined temperature range. Thepredetermined temperature range may, for example, be below thetemperature at which the build material would experience bonding in thepresence of coalescing agent 504. For example, the predeterminedtemperature range may be between about 155 and about 160 degreesCelsius, or the range may be centered at about 160 degrees Celsius.Pre-heating may help reduce the amount of energy that has to be appliedby the energy source 226 to cause coalescence and subsequentsolidification of build material on which coalescing agent has beendelivered or has penetrated.

At 312, as shown in FIGS. 5b and 6b , coalescing agent 504 and 506 maybe selectively delivered in patterns to the surface of portions of thelayer 502 b in accordance with the modified data 412. As discussedearlier, the agent 504 and 506 may be delivered by agent distributor202, for example in the form of fluids such as liquid droplets.“Selective delivery” means that agent may be delivered to selectedportions of the surface layer of the build material in various patterns.

The modified data 412 may include slice 402 defining a slice that is tobecome solid to form part of the three-dimensional object beinggenerated using coalescing agent 504, as shown in FIGS. 5b and 6b . Insome examples, to form the slice the coalescing agent 504 is to bedelivered at a density or amount sufficient to cause coalescence andsolidification of build material upon application of energy, e.g. due tothe coalescing agent 504 acting as an absorber of applied energy tofacilitate the coalescence and solidification. In some examples, theagent distributor 202 may be to deliver drops of coalescing agent 504 ata density of between about 0.5 to 2 drops (e.g. 1 drop) per 1/600× 1/600inch region ( 1/360000 square inches). Each drop may have a mass ofabout 5 nanograms to about 20 nanograms. In other examples the agentdistributor 202 may be to deliver drops of coalescing agent 504 at ahigher or lower density sufficient to achieve coalescence andsolidification.

The modified data 412 may also include portion 414 defining a lowerdensity or amount of coalescing agent 506 to be delivered relative tothe density or amount of coalescing agent 504 to be delivered togenerate the slice defined by slice 402 of modified data 412. The lowdensity coalescing agent 506 may be delivered in pattern surrounding theboundary of the thin portion generated using the coalescing agent 504.In some examples, the coalescing agent 506 may act as an absorber ofapplied energy, and the lower density of the coalescing agent 506relative to the coalescing agent 504 may have the following effects. Thebuild material on which the coalescing agent 506 is applied may notcoalesce and solidify upon application of energy, or may minimallycoalesce and solidify such that any slightly solidified portions may notform a permanent part the object being generated (e.g. the slightlysolidified portions may be shaken off the formed object). Thus, thedensity of coalescing agent 506 may be insufficient to achieve fullsolidification in the second portion when energy is applied.Additionally, the build material on which coalescing agent 506 isapplied may prevent at least some heat from flowing away from the thinportion 414 of the slice generated by applying coalescing agent 504 onthe build material. This may be due to a reduced thermal gradientbetween the portions having coalescing agent 504 and portions havingcoalescing agent 506, due to the portion having coalescing agent 506being closer to the temperature of the portions having coalescing agent504 than would be the case compared to if the coalescing agent 506 werenot applied. In some examples, the agent distributor 202 may, forexample, be to deliver drops of coalescing agent 506 at a density ofbetween about 1/128 to 1/32 drops (e.g. 1/64 drops) per 1/600× 1/600inch region ( 1/360000 square inches). Each drop may have a mass ofabout 5 nanograms to about 20 nanograms. In some examples, the densityof coalescing agent 506 to be delivered may be at least an order ofmagnitude less than the density of coalescing agent 504 to be delivered.In some examples, the density of the coalescing agent 506 may depend onthe thickness or the portion (e.g. thin portion). For example, a firstthin portion of the object that is thinner than a second thin portion ofthe object may be have a surrounding portion of coalescing agent 506 ofa greater density than a density of coalescing agent of a surroundingportion of the second thin portion.

In other examples the agent distributor 202 may be to deliver drops ofcoalescing agent 506 at a higher or lower density sufficient to cause noor minimal solidification where coalescing agent 506 is delivered but toprevent at least some heat from flowing away from the thin portion 414.

In some examples, the same coalescing agent distributor 202 may be todeliver the different densities of coalescing agent 504 and 506, but inother examples, different coalescing agent distributors may be todeliver the different densities of coalescing agent 504 and 506.

In some examples, coalescence modifier agent may similarly beselectively delivered in patterns to portions of the layer 502 bsurrounding the portions having low density coalescing agent 506. Insome examples, coalescence modifier agent may also be selectivelydelivered in patterns to a portion of the layer 502 b surrounding thickportions of the slice having higher density coalescing agent 504. Thecoalescence modifier agent may be to reduce coalescence bleed, which issolidification of portions of the build material that are not intendedto be solidified. Coalescence bleed may result, for example, in areduction in the overall accuracy of generated three-dimensionalobjects.

FIGS. 5c and 6c show coalescing agent 504 and 506 having penetratedsubstantially completely into the portions of the layer 502 b of buildmaterial, but in other examples, the degree of penetration may be lessthan 100%. The degree of penetration may depend, for example, on thequantity of agent delivered, on the nature of the build material, on thenature of the agent, etc.

At 314, a predetermined level of energy may be temporarily applied tothe layer 502 b of build material. In various examples, the energyapplied may be infra-red or near infra-red energy, microwave energy,ultra-violet (UV) light, halogen light, ultra-sonic energy, or the like.In some examples, the energy source may be focused. In other examples,the energy source may be unfocused. The temporary application of energymay cause the portions of the build material on which the higher densityof coalescing agent 504 was delivered to heat up above the melting pointof the build material and to coalesce, and may cause the portions ofbuild material on which the lower density of coalescing agent 506 wasdelivered to not coalesce or minimally coalesce. For example, thetemperature of some or all of the layer 502 b may achieve about 220degrees Celsius. Upon cooling, the portions having coalescing agent 504may coalesce may become solid and form part of the three-dimensionalobject being generated, as shown in FIG. 5d and FIG. 6 d.

As discussed earlier, one such solidified portion 508 may have beengenerated in a previous iteration. The heat absorbed during theapplication of energy may propagate to the previously solidified portion508 to cause part of portion 508 to heat up above its melting point.This effect helps creates a portion 510 that has strong interlayerbonding between adjacent layers of solidified build material, as shownin FIG. 5d . The portions having coalescing agent 504 may also achievegood object properties such as strength good surface properties,accuracy, strength, and inter-layer bonding, as a result of the portionshaving coalescing agent 506 acting to prevent at least some heat fromflowing away from the portions having coalescing agent 504.

After a layer of build material has been processed as described above in310 to 314, new layers of build material may be provided on top of thepreviously processed layer of build material. In this way, thepreviously processed layer of build material acts as a support for asubsequent layer of build material. The process of 310 to 314 may thenbe repeated to generate a three-dimensional object layer by layer.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the elementsof any method or process so disclosed, may be combined in anycombination, except combinations where at least some of such featuresand/or elements are mutually exclusive.

In the foregoing description, numerous details are set forth to providean understanding of the subject disclosed herein. However, examples maybe practiced without some or all of these details. Other examples mayinclude modifications and variations from the details discussed above.It is intended that the appended claims cover such modifications andvariations.

What is claimed is:
 1. A system for generating a three-dimensionalobject, the system comprising: at least one agent distributor toselectively deliver coalescing agent onto portions of a layer of buildmaterial at a first density and at a second density lower than the firstdensity; and a controller to control the agent distributor toselectively deliver the coalescing agent at the first and seconddensities onto respective first and second portions of the layer ofbuild material in respective first and second patterns derived from datarepresenting the three-dimensional object to be generated, so that whenenergy is applied to the layer the build material is to coalesce andsolidify to form a slice of the three-dimensional object in accordancewith the first pattern, wherein the second portion is disposed around aperimeter of the layer of build material, wherein presence of thecoalescing agent in the second portion is to prevent at least some heatfrom flowing away from the first portion when the energy is applied. 2.The system of claim 1, wherein the coalescing agent is an ink. Thesystem of claim 1, wherein the at least one agent distributor is apage-wide distributor.
 4. The system of claim 1 wherein the seconddensity is selected based on a thickness of the first portion.
 5. Thesystem of claim 1, wherein the second portion comprises a regionsurrounding the first portion.
 6. The system of claim 1, wherein the atleast one agent distributor distributes coalescing agent at a density tocause reduced solidification where coalescing agent is delivered andprevent heat from flowing away from the first portion.
 7. A system forgenerating a three-dimensional object, the system comprising: a firstagent distributor to selectively deliver coalescing agent onto portionsof a layer of build material at a first density; a second agentdistributor to deliver coalescing agent onto the layer of build materialat a second density, wherein the second density is at least an order ofmagnitude less than the first density; and a controller to controlapplication of the coalescing agent by the first agent distributor andthe second agent distributor.
 8. The system of claim 7, furthercomprising a preheater to preheat the layer of build material prior tocoalescence of portions of the layer of build material.
 9. The system ofclaim 7, wherein the first agent distributor applies coalescing agent toregions of the layer of build material which are to coalesce underradiation.
 10. The system of claim 7, wherein the second agentdistributor applies the coalescing agent uniformly to the layer of buildmaterial.
 11. The system of claim 7, further comprising a radiationsource to uniformly apply radiation to the layer of build material,wherein radiation from the radiation source is absorbed by coalescingagent deposited on the layer of build material.
 12. The system of claim7, wherein the radiation source moves across the layer of buildmaterial.
 13. The system of claim 7, further comprising a third agentdistributer to selectively deliver a coalescence modifier agent adjacentto regions receiving the coalescing agent at the first density having athickness above a threshold.
 14. A method comprising: receiving athree-dimensional model of an object to be generated, thethree-dimensional model comprising slice data for each slice of theobject to be generated; modifying the slice data to add a boundaryportion around a thin portion of the slice, wherein the boundary portionis to receive a lower amount of coalescing agent as compared toremaining portions of the slice such that the boundary layer reducesoutflow of heat from the thing portion during irradiation; and formingthe slice according to the slice data.
 15. The method of claim 14,further comprising adding a boundary portion around an entirecircumference of the slice.
 16. The method of claim 14, whereinmodifying the slice data comprises dilating a boundary of the thinportion having a thickness below a threshold.
 17. The method of claim16, wherein the thin portion boundary is dilated to the thickness of theboundary portion.
 18. The method of claim 14, further comprisingpreheating a layer of build material prior to irradiating the layer ofbuild material.
 19. The method of claim 14, wherein the thin portion hasa thickness less than 2 millimeters.
 20. The method of claim 14, furthercomprising delivering a coalescence modifier agent to reduce coalescencebleed.